When a transmitting device and a receiving device communicate media information such as images, voices and the like with each other on a packet network such as an IP (Internet protocol) network or the like, sometimes real-time transmission and non-real-time transmission are mixed.
In real-time transmission, for example, as illustrated in FIG. 1, image signals obtained by a monitor camera 1305 are encoded by a real-time image distribution device 1303, are transmitted to a real-time image receiving device 1301 as a real-time motion image packet, and are decoded/reproduced. In order to synchronize the clock of image signals to be decoded/reproduced by the real-time image receiving device 1301 with a clock at the time of encoding in the real-time image distribution device 1303, for example, a method using PCR (program clock reference) is adopted. In this method, a clock synchronization packet called PCR is multiplexed on a motion image packet to be transmitted. On the decoding side, a difference between the received PCR data and a system time clock (STC) at the time of reception on the decoding side is calculated. Then, a voltage-controlled crystal oscillator (VCXO) is controlled by this differential value and a reproduction clock is generated. Specifically, in the real-time transmission, the size of a packet receiving buffer on the real-time image receiving device 1301 side is reduced as much as possible by synchronizing the clocks of the real-time image distribution device 1303 (encoder) and the real-time image receiving device 1301 (decoder) and the delay of images can be avoided as much as possible.
However, in non-real-time transmission, as illustrated in FIG. 1, an encoded image signal file accumulated in the storage device of a file distribution server 1304 is read and transmitted using a packet in response to a request from a server distribution image receiving device 1302. The server distribution image receiving device 1302 decodes/reproduces the image signal file transmitted using a packet on the basis of a system clock in its own device. In non-real-time transmission, it is technically difficult to transmit an image signal file at the same timing as that at the time of encoding. Therefore, the server distribution image receiving device 1302 exercises flow control in order to absorb a difference between reproduction timing and transmission timing. In the flow control, when a packet remaining in the packet receiving buffer seems to exceed the buffer size, a transmit stop request packet is transmitted from the server distribution image receiving device 1302 to the file distribution server 1304 and the file distribution server 1304 stops the transmission of an image signal file. Conversely, when no packet seems to be remaining in the packet receiving buffer, a transmit start request packet is transmitted from the server distribution image receiving device 1302 to the file distribution server 1304 and the file distribution server 1304 starts (re-starts) the transmission of an image signal file.
In this case, although the real-time image receiving device 1301 illustrated in FIG. 1 usually receives image signals transmitted from the real-time image distribution device 1303 in real time, there has been a desire for the real-time image receiving device 1301 to also be able to receive image signals transmitted from the real-time image distribution device 1303 not in real time.
In order to realize such mixed reception, it is necessary for the real-time image receiving device 1301 to have a flow control function for non-real-time transmission.
However, when the real-time image receiving device 1301 attempts to receive an image signal file transmitted from the real-time image distribution device 1303 not in real time, there is no prior art capable of discriminating real-time transmission from non-real-time transmission. Therefore, conventionally a flow control function cannot be mounted on the real-time image receiving device 1301.
Therefore, when the real-time image receiving device 1301 receives an image signal file transmitted from the real-time image distribution device 1303 not in real time without flow control, as illustrated in FIG. 1, there is a high possibility that the difference between reproduction timing and transmission timing may increase. As a result, an overflow or underflow of packets occurs in the packet receiving buffer of the real-time image receiving device 1301 and the image signal file cannot be normally received, which is a problem.